Grey seals in the dock over porpoise murder mystery

It’s one of the big mysteries in my career as a marine biologist. Something lurking in the seas off Britain has been chomping away at local porpoises and none of the usual suspects fit the bill. Now scientists have finally identified the cuddly culprit.

I first became aware of the attacks on porpoises – beakless, smaller relatives of dolphins – as I run the UK shark tagging programme and am often asked to comment on possible sightings. In both 2010 and 2011 I was sent pictures from the Norfolk coast of porpoise carcasses with a considerable amount of tissue bitten away.

The common assumption was that these were due to shark attacks. However none of the photographs of the wounds showed the characteristic punctures caused by the multiple rows of shark teeth, such as displayed by human victims of shark attacks.

The UK isn’t exactly known for its deadly sea beasts. While a number of shark species do live round the coast and in the North Sea, the most common are too small to inflict the bites we were dealing with. Larger sharks capable of removing that amount of tissue are very rare.

Invariably the popular press claims attacks like these are evidence of great white sharks. After the 2011 find near Great Yarmouth, Norfolk, the Daily Mail thought a “giant shark or killer whale could be stalking the coast”, The Sun even said it might be Jaws himself.

Given that there are no fully substantiated reports of white sharks in UK waters and that in any case such a shark could quickly dispose of a whole porpoise, this is fanciful.

But at last, we have a more likely explanation.
Whodunnit?

The unfortunate Norfolk porpoises aren’t alone. In fact, huge numbers of harbour porpoises have been washing up on the shores of the North Sea. They shared the same nasty-looking bite marks.

A team of Dutch scientists has now identified grey seals as the culprits. Their work, reported in the Royal Society journal, reveals DNA from grey seals has been found in bite marks on porpoise carcasses.

The researchers examined 721 dead porpoises in detail. The sheer numbers enabled them to identify the key characteristics of seal bites including substantial loss of skin and blubber, puncture wounds (often repetitive) to the head, tail and flippers, plus series of parallel scratches anywhere on the body left by seal claws.

A flow chart has been produced using these marks to determine if the porpoise was attacked by a seal and if it could have escaped. This will be invaluable to those undertaking autopsies of porpoises in the future.

The majority of confirmed seal attacks are on juveniles. Since many of the porpoise carcasses were found on Dutch bathing and surfing beaches the authors offer up the thought that humans may be at risk.

Perhaps this serves us right, as it seems humans may have triggered this change in seal diet. Along with their close relatives dolphins and whales, porpoises often become entangled in fishing gear and some attacks may simply be seals scavenging trapped porpoises. Having got used to the scavenged fare, the authors speculate that seals may have turned to attacking live porpoise.

Porpoises have a hard life. Even dolphins attack them. They are victims of the fishing industry and subject to an increasing amount of noise from boats, oil rigs and wind turbines.

Now, possibly triggered by our activities, there is yet another pressure on this species: hungry seals.